At the passage of the shock, the DCN abundance increases by a factor 7 whereas DNC remains unchanged. Overall, the magnitude of these variations remain small in front of those affecting the main isotopologues and indicate that, at first order, the abundances of these deuterated species are only moderately affected by the passage of the shock, with a different behaviour for DCN and DNC.
As for L1157-B1, previous estimates of the DCN/HCN ratio were obtained by [1]}, based on a reduced number of transitions of H\(^{13}\) CN and DCN. Our present determination of \((3.1\pm 1.0)\times 10^{-3}\)  is consistent with their previous estimate D/H = (0.5–3)\(\times 10^{-3}\) . More recently, [2]} reported similar deuterium fractionation ratios (\(\approx 4.0\times 10^{-3}\) ) in the small-scale structures detected in the bow-shock, with no significant spatial variation.
Our unbiased analysis of the deuterated isomers confirms that the molecular D/H ratio decreases across the shock and it allows us to quantify the magnitude of the effect: a factor 26 and 8 for HCN and HNC, respectively.
